Comprehensive smoke-free policies such as those called for by the WHO FCTC are the only way to protect the public effectively from the harms of secondhand smoke (SHS), yet Japan has been slow to implement this important health measure. This study examines baseline levels of smoking and SHS exposure in public places and support for smoking bans in Japan prior to the implementation of the 2018 national smoke-free law. Data are from the International Tobacco Control (ITC) Japan Wave 1 Survey (Feb–Mar 2018), a web survey of adult cigarette smokers, heated tobacco product users, dual users, and non-users (total N = 4684). Measures included prevalence of smoking (whether respondents noticed people smoking inside restaurants and bars at their last visit, and workplaces in the last month), and support for complete smoking bans in these venues. Smoking prevalence in each venue was high overall in 2018 (49% of workplaces, 55% of restaurants, and 83% of bars), even higher than in China, the country with the greatest toll of SHS. Support for complete smoking bans was very high overall (81% for workplaces, 78% for restaurants, and 65% for bars). Non-users were less likely to be exposed to SHS and had higher support for smoking bans than tobacco users. These findings point to the ineffectiveness of partial smoke-free laws in Japan and reinforce the call for comprehensive smoke-free laws, which even smokers would support at higher levels than in many other ITC countries.